Step 2: Choosing the Orbit

Every builder knows the rule: location, location, location. For a planet, location means its orbit — how far it sits from its star and what path it follows. Get this wrong and no amount of later work will make the planet comfortable.

Too hot, too cold, just right

The amount of starlight a planet receives drops quickly with distance. Double the distance and the planet gets only a quarter of the light. Move Earth to where Venus orbits and it would receive almost twice as much sunlight; move it out to Mars and it would get less than half.

The range of distances where a planet with the right atmosphere could keep liquid water on its surface is called the habitable zone, sometimes nicknamed the Goldilocks zone.

• Inner edge: closer than this, oceans evaporate and water vapour — itself a greenhouse gas — traps more heat until the oceans boil away. This is called a runaway greenhouse, and it is probably what happened to Venus.
• Outer edge: farther than this, even a thick carbon dioxide atmosphere cannot keep the surface above freezing.

For our Sun, a commonly used conservative estimate puts the habitable zone at roughly 0.95 to 1.7 times Earth's distance. Earth sits comfortably inside it; Venus is just inside the inner edge; Mars is near the outer edge. You can try other stars in the habitable zone calculator.

Why temperature is not just distance

If you calculate Earth's temperature from distance alone, assuming it reflects about 30% of sunlight like it does today, you get roughly −18 °C. Earth's actual average is about +15 °C. The 33-degree difference comes from the greenhouse effect of water vapour and carbon dioxide.

So a planet's climate depends on three things working together: how much light arrives, how much is reflected (its albedo), and how much heat the atmosphere keeps.

The shape of the orbit

Orbits are ellipses, not perfect circles. Earth's orbit is nearly circular — our distance from the Sun changes by only about 3% over a year. A planet on a stretched, eccentric orbit would swing between scorching summers and deep freezes every year. For a stable climate, a nearly round orbit is the safer choice.

Orbits also set the length of the year. Johannes Kepler found that planets farther out move more slowly and have much longer paths, so their years grow quickly with distance: Mars takes 1.9 Earth years to go around the Sun, Jupiter 11.9 and Neptune about 165.

Planets move house

One of the surprises of the past three decades is that planets do not always stay where they formed. Interactions with the gas disk and with other planets can push them inward or outward — a process called migration.

• The first planet found around a Sun-like star, 51 Pegasi b (discovered in 1995), is a giant planet orbiting closer to its star than Mercury is to the Sun. Giants cannot form that close, so it almost certainly migrated inward. Planets like this are called hot Jupiters.
• In our own solar system, computer models suggest the giant planets shifted their orbits early on. One idea, the Grand Tack hypothesis, proposes that Jupiter drifted inward and then back out, which could explain why Mars is so small. These models are still debated.

For a planet builder, migration is both a risk and a tool: a world can be assembled in one place and end up somewhere else entirely.

Neighbours matter

A good orbit is also a stable one. If two planets pass too close, their gravity can throw one of them onto a wild path or out of the system altogether. Stable planetary systems tend to keep their planets well spaced. In our solar system, Jupiter's gravity also shaped the asteroid belt and keeps stirring it — which affects how often rocks are sent towards the inner planets.

Builder's checklist for step 2

• Inside the star's habitable zone if you want surface water
• A nearly circular orbit for steady seasons
• Enough distance from neighbouring planets to stay stable
• Awareness that the planet may migrate before it settles

The site is chosen. Now it is time to pour the foundations: Step 3: Building the core and crust.